I think the most common grammatical error I come across in my proof-reading or marking of young people's work these days is finding should of used instead of should have.
You can see how it happened. Instead of saying should have most of us shorten it to should've. Those who learn their English as children hear it as an abbreviation of should of which they repeat at school. Uncorrected it sticks.
It litters the Twitterverse. That may just be my favourite sentence of the year so far.
The evolution of language is an interesting thing. Most words have etymorphised, if you'll excuse me inventing a word the meaning of which is obvious.
I'm only Tilley because, apparently, the Cornish commoner couldn't cope with Tillé, which is French for labourer and probably where I come from. Should of known better.
À tout de suite. Or something.